With arising of large-capacity service, an optical communication backbone network has a tendency to evolve from the rate of 10 Gbit/s to 40 Gbit/s and higher and at the same time the channel spacing gradually evolves from 100 GHz to 50 GHz. Moreover, opto-electrical conversion in the link tends to decrease, which makes it more difficult to directly detect the bit error rate at the electrical layer; while detecting the bit error rate at the link terminal interferes with fault location. As commercial networks widely adopt a rate of 40 Gbit/s and higher, it becomes increasingly important to perform Optical Performance Monitoring (Optical Performance Monitoring, OPM) on Dense Wavelength Division Multiplexing (Dense Wavelength Division Multiplexing, DWDM) signals to effectively control and manage optical networks. The Optical Signal to Noise Ratio (Optical Signal to Noise Ratio, OSNR) may reflect the signal quality accurately and thus is an important performance indicator required to be detected.
The traditional OSNR detection uses the linear interpolation method in which the inband noise is estimated by measuring the inter-channel outband noise and then the OSNR is calculated. This method is effective for simple low-speed point-to-point DWDM networks. However, with wide use of optical sub-systems such as the Reconfigurable Optical Add-Drop Multiplexer (ROADM), which carry the optical filter, the outband noise between channels is restricted by filtering and is smaller than the inband noise in the wavelength of the actual channels, resulting in inaccuracy for the method which uses the outband noise to calculate the inband noise, and higher OSNR value detected.
In addition, for high-speed DWDM networks with the rate of 40 Gbit/s and higher, on one hand, because of broad signal spectrum width, the filter fails to cover all signal powers at the time of obtaining signal power, resulting in lower detection power; on the other hand, because of small space between channels, signals of the channel or adjacent channels cross into the filter at the time of measuring the outband noise and are mistaken as noise, resulting in higher value of detected noise. The two comprehensive effects cause the OSNR value to be relatively small.
To solve the problem that outband OSNR detection is inaccurate, inband ONNR detection will play an important role in next-generation optical networks.
Two inband OSNR detection methods are adopted in the prior art, wherein the signals involved are polarized light, and unpolarized light features Amplified Spontaneous Emission (Amplified Spontaneous Emission, ASE). At the detection point, the light to be detected passes through a polarization controller and then through a polarization beam splitter or two vertical linear polarizers. By continuously adjusting the polarization controller to change the polarization state of signals, the maximum and minimum values of light intensity on two output ports are obtained. When the polarization state of signals is in the same polarization direction with the linear polarizer, the signals can pass completely, but only a half of the noise, whose polarization state is in the same polarization direction with the linear polarizer, can pass. In this case, the light intensity is at the maximum, being the signal power plus a half of noise power.
                              P          max                =                              P            S                    +                                    1              2                        ⁢                          P              N                                                          (        1        )            
Similarly, when the polarization direction of the polarization state of signals is perpendicular to that of the linear polarizer, the light intensity is at the minimum, which is only a half of the noise power.
                              P          min                =                              1            2                    ⁢                      P            N                                              (        2        )            
With this method, inband OSNR detection is implemented.
                    OSNR        =                                            P              S                                      P              N                                =                                                    P                max                            -                              P                min                                                    2              ⁢                              P                min                                                                        (        3        )            
In implementation of the preceding inband OSNR detection, the inventor finds that the prior art has at least the following problems.
1. An expensive high-speed polarization controller is required to scan the polarization state, which leads to a high detection cost.
2. The polarization states of signals in each channel are different, and thus polarization state scanning needs to be performed on all channels, which results in a slow detection speed.
3. The method is based on the assumption that signals are in a single polarization state, and thus is incapable to be used in the Polarization Division Multiplexing (Polarization Division Multiplexing, PDM) system. However, the PDM system will be widely adopted in the future 100 Gbit/s high-speed system.
In a word, the polarization devices in the prior art are high in cost and slow in scanning speed, and is not applicable to the PDM system, and the DWDM systems with the rate of 40 Gbit/s or higher, 50 GHz spacing and signal bandwidth close to channel bandwidth.